EP0376500B1

EP0376500B1 - Process for the spectral sensitisation of a silver halide emulsion

Info

Publication number: EP0376500B1
Application number: EP89312508A
Authority: EP
Inventors: Rolf Steiger
Original assignee: Ilford AG
Current assignee: Ilford Imaging Switzerland GmbH
Priority date: 1988-12-27
Filing date: 1989-11-30
Publication date: 1994-04-13
Anticipated expiration: 2009-11-30
Also published as: JP2841220B2; US5077190A; DE68914626D1; DE68914626T2; JPH02264245A; EP0376500A1

Description

The present invention relates to a process for spectral sensitisation of a silver halide emulsion and to photographic materials produced therewith.
For spectral sensitisation of silver halide emulsions, the sensitising dye is in general adsorbed on the surface of the silver halide crystals after the chemical sensitisation. However, it is also usual to carry out the spectral sensitisation simultaneously with or before the chemical sensitisation. Thus, a process is described in US-A-4,225,666, in which a part of the sensitising dye is added during, and the remainder is added after the formation of the silver halide crystals. From EP-A-0,069,596, it is known to add a part of the sensitising dye during the chemical sensitisation and a second part of the sensitising dye or a corresponding quality of another dye after the chemical sensitisation. In DE 2402130 a method of preparing an internally sensitive silver halide emulsion is described wherein further silver halide is laid down on the grains of a surface sensitive or surface fogged emulsion so that excesses of silver and halide ions are alternately produced.
It has now been found that the spectral sensitisation of silver halide emulsions can be improved further, if the sensitising dye is adsorbed in a more useful manner on the surface of the silver halide crystals.
Therefore, according to the present invention there is provided a process for the spectral sensitisation of a silver halide emulsion characterised in that:

A) (1) A silver salt solution is added to a preformed silver halide emulsion until the pAg value reaches a point close to equivalence, (2) the pAg value of the emulsion is then adjusted to a value between 7.5 and 10, by addition, of an ammonium or alkali metal halide or pseudo-halide solution to achieve fractional coverage of newly deposited silver halide (3) a part quantity of a predetermined quantity of a solution of a sensitising dye capable of forming a J band is absorbed on the newly deposited silver halide and:-
B) Stages (1), (2) and (3) are repeated at least once more but up to seven times, the residual quantity of sensitising dye being adsorbed with the last repeat of stage (3).

Usefully in stage (2) the pAg value of the emulsion is adjusted to a value between 7.5 and 8.5.
Most usually the stages (1) to (3) are repeated from 3 to 5 times.
By pseudo-halide is meant an anion which forms an insoluble silver salt which can co-precipitate with a water-soluble halide and which can react with silver nitrate to form water-insoluble crystals or parts of a crystal. In the photographic system a pseudo-halide acts in a similar manner to a halide and can be used to replace or partially replace a halide.
Examples of pseudo-halides are alkali metal or ammonium thiocyanate, or alkali metal or ammonium cyanide.
The preferred pseudo-halides for use in the process of the present invention are alkali metal thiocyanates.
By J-Band aggregating dyes are meant cyanine dyes which self-aggregate producing shifts to longer wavelength and sharper absorption curves than the non-aggregated dyes. Such J-band aggregation is described in The Theory of the Photographic Process by James, 4th Edition, 1977 at pages 218-222.
In the process of the present invention the J-Band aggregating dyes are added to the colloid dispersion, either as an aqueous solution or as a dispersion in an aqueous medium.
Water-miscible solvents are usually present in the aqueous solution of the J-band aggregating dyes such as methanol, ethanol or acetone. By preformed silver halide emulsion is meant an emulsion which comprises in a colloid dispersion medium silver halide grains which are of sufficient size to be usable in a photosensitive assembly.
Most preferably such silver halide grains have been chemically sensitised.
By chemically sensitised is meant the increase in light-sensitivity of the silver halide grains by the action of certain chemicals such as reducing agents, gold and sulphur compounds. A description of chemical sensitisation is given in The Theory of the Photographic Process by James, 4th Edition (pages 149-158).
The preferred chemical sensitisation for the emulsion of the present invention is a combination of sulphur and gold sensitisation.
For the chemical sensitisation any of the known procedures can be used, for example procedures which are described in RD No 17643, December 1978 and No 22534, January 1983 and in H Frieser, "Die Grundlagen der Photographischem Prozesse mit Silberhalogeniden", pages 675-734, (Akademische Verlags Gesellschaft 1968).
More specifically, sulphur sensitisation methods using active gelatin, and compounds containing sulphur capable of reacting with silver ions (eg thiosulphates, thioureas, mercapto compounds, and thiocyanates), reduction sensitisation methods using reducing materials (eg stannous salts, amines, hydrazine derivatives, formamidine, sulphinic acid and silane compounds), noble metal sensitisation methods using noble metal compounds (eg gold compounds and complex salts of Group VIII metals such as platinum, iridium and palladium) and so on can be employed independently or in combination.
The present invention also relates to a photographic material which contains, on a base, at least one silver halide emulsion layer spectrally sensitised according to the process defined above.
For carrying out the process according to the invention, a silver salt solution such as, for example, a silver nitrate solution is first added to a silver halide emulsion until the pAg value of the emulsion reaches a point close to equivalence. It is disadvantageous to adjust to an excess of silver ions. Those pAg values are preferred which are one to two units above the point of equivalence. The pAg value of the emulsion is then increased to a value between 7.5 and 10 by addition of an alkali metal halide or thiocyanate solution, which is preferably used. As a result of these two measures, terraces are formed on the surface of the silver halide crystals of the emulsion, where the sensitising dye is preferentially attached.
In the process according to the invention, after the two said measures, only a part of the quantity of sensitising dye required for optimum sensitisation of the silver halide emulsion is initially added. The adjustment of the pAg values is then repeated and the residual quantity of sensitising dye is not supplied until then.
In order to obtain optimum results, the steps of adjusting the pAg values and adding the part quantity of sensitiser can, if desired, be repeated up to six times, preferably once to three times, before the residual quantity of sensitiser is then added to the silver halide emulsion in the seventh or, respectively, second, third or fourth pass. Even more repeats of these steps do not bring any further advantage.
The size of the part quantity depends essentially on the number of repeats to be carried out and corresponds approximately to the total quantity of sensitiser to be used, multiplied by the number of repeats and divided by a number between 3 and 10, preferably 8 to 10.
The number of repeats which are optimum for the particular silver halide emulsion must be established by experiments beforehand.
It is thought that in the process of the present invention the fractional coverage of the newly deposited silver salt on the preformed silver halide crystals takes the form of terraces of atomic dimensions not visible by transmission electron microscopy. It is thought that these terraces are separated from each other by steps of high surface energy. The J-Band aggregating dye nucleates on these steps and is deterred from recombining. At the end of the process, frequently the whole surface of the silver halide crystals is covered with such terraces and steps on which the J-Band dyes have nucleated. When such crystals are photo exposed, electron hole recombination is decreased leading to higher photographic speed and low LIRF.
The silver halide emulsions which can be sensitised according to the invention can consist, for example, of silver chloride, silver bromide or mixed silver halides such as silver chlorobromide, silver iodobromide or silver chloroiodobromide. These emulsions can be prepared by known methods such as are described, for example, in RD 17643 (December 1978) or RD 22534 (January 1983).
Sensitising dyes suitable for carrying out the process according to the invention are of the general formula

in which R₁ and R₂ independently are a substituted or unsubstituted alkyl or aryl radical, L₁, L₂ and L₃ independently are a substituted or unsubstituted methine residue, Z₁ and Z₂ independently are an atom or a group of atoms which are required to complete a 5-membered or 6-membered, substituted or unsubstituted heterocyclic nucleus, m₁ and m₂ independently are 0 or 1, n₁ is 0, 1 or 2, X is an anion and l is 1 or 2, with the proviso that l is 1 if the compound forms an inner salt.
Examples of preferred sensitising dyes which can be used for the process according to the invention are:

The preformed silver halide emulsion used in the present invention can be comprised of silver bromide, silver chloride, silver chlorobromide, silver chloroiodide, silver bromoiodide, silver chlorobromoiodide or mixtures thereof. The emulsions can include coarse, medium or fine silver halide grains and they may have regularly shaped, for example, cubic or octahedral, crystals or they may have irregularly shaped, for example, spherical or tabular, crystals. Alternatively, the crystals may be combinations of these shapes. The grains may be a mixture of ones having various crystal shapes. The grains may have different phases between the interior and the surface, or they may possess a uniform phase.
The preformed photographic emulsion used in the present invention can be prepared by any of the methods described in P Glafkides, "Chimie et Physique Photographique", Paul Montel, 1967, GF Duffin "Photographic Emulsion Chemistry", The Focal Press, 1966 and V L Zeilman et al "Making and Coating Photographic Emulsion", The Focal Press, 1964. That is, the emulsion may be prepared by an acid process, neutral process or ammonia process. The "controlled double-jet method" wherein the solution for forming silver halide grains is kept at a constant level of pAg is preferably used as this method is effective for producing a silver halide emulsion comprising grains of a substantially uniform size having regular crystal shapes.
Other methods for the preparation of silver halide emulsion which can be used for the present invention are described, for example, in Research Disclosure No 17643, December 1978, RD No 22534, January 1983 and RD No 23212, August 1983.
The preformed silver halide emulsion used in the present invention may contain other compounds, such as azo dyes, colour couplers, optical brightening agents, UV-absorbers, filter dyes, stain inhibitors, stabilisers, hardeners, coating aids and antistatic agents. Such additives are described for example in RD No 17643, December 1978.
The finished emulsion as prepared by the method of the present invention may be coated on an appropriate support such as baryta paper, resin-coated paper, synthetic paper, triacetate film, polyethylene terephthalate film or a glass plate. Various coating methods, including dip coating methods, an air knife coating, cascade coating, curtain coating and an extrusion coating method can be employed.
Such a support may be either transparent or opaque depending upon the intended use of the light sensitive material. When a support used in transparent, it can be colourless or coloured by addition of a dye or a pigment
Photographic light sensitive material to which the emulsion of the present invention can be applied include various colour and black and white photosensitive materials. Specific example of such materials include colour negative films (for amateur use, motion picture use, etc), colour reversal films (for slide use, motion picture use, etc), colour photographic paper, colour positive films (for motion picture use etc), colour reversal photographic papers, heat-developable colour photosensitive materials, colour photosensitive materials for a silver dye bleach process, photographic light-sensitive materials for a photomechanical process (lith films, scanner films, etc), X-ray photographic light sensitive materials (for medical use employing radiography or fluorography, for industrial use etc), black and white negative films, black and white photographic papers, microphotographic light sensitive materials (COM, microfilms etc), colour diffusion transfer photosensitive materials (DTR), silver salt diffusion transfer photosensitive materials and printout photosensitive materials.
In the photographic processing of a photosensitive material to which the silver halide emulsion prepared in accordance with the present invention is applied, any known processing method and any known processing solution can be employed. The processing temperature is generally in the range of about 18°C to about 50°C. Of course, temperatures lower than about 18°C or higher than about 50°C may be employed. The photographic processing may include either development processing for forming a silver image (black and white photographic processing) or development processing for forming a dye image (colour photographic processing).
Preferably when the emulsions of the present invention are present in a photographic assembly which has been photographically exposed the assembly is developed in a solvent developer. By solvent developer is meant a developer which comprises a proportion of a silver halide solvent and thus is able to develop an internal latent image in the silver halide crystals. Examples of useful silver halide solvent are thiosulphates and thiocyanates.
The following Examples will serve to illustrate the invention. In these Examples % means by weight.

Example 1

83g of a 10% aqueous gelatin solution and 40g of a polydisperse silver bromoiodide emulsion are mixed with one another at 40°C (emulsion mixture A). The silver halide emulsion contains 71g of gelatin/kg and 57g of Ag/kg as silver bromoiodide with 5.2 mol% of iodide, and the mean particle size is 0.55µm. The silver halide emulsion has been chemically sensitised with thiosulphate and gold thiocyanate to the optimum speed.
Silver nitrate solution, potassium bromide solution and a solution of the sensitiser of the formula (22) as indicated in Table 1 are added to this mixture successively at 40°C. This treatment is repeated, the residual quantity of the sensitiser solution required for optimum sensitisation being added at the end of the last repeat. The number of repeats can be seen from Table 1.
At the end of the cycles, 2.6ml of a 1% solution of 5-methyl-7-hydroxy-2,3,4-triazaindolizine, 2ml of 8% solution of Nekal BX and 94ml of water are added. 100ml of this mixture are coated per m² of a transparent polyester base, together with a gelatin supercoat which contain 1.5g of gelatin per m² and 48mg of the gelatin hardener, 2,4-dichloro-6-hydroxy-triazine (potassium salt).
(Nekal is the registered trade mark of G.A.F. Corp.)
For comparison, the same emulsion is spectrally sensitised once without a pAg cycle (comparison 1) and once only after 4 pAg cycles have been carried out. In this case, the cycles are carried out analogously to experiment No 3, but the total quantity of the sensitiser is not added until the end of the 4 cycles. Table 2 shows the sensitometric results of the coated samples after exposure and development for one minutes in Developer A and subsequent fixing.

Developer A

Ethylenediaminetetraacetic acid (sodium salt)	4.0 g
Potassium sulphite	19.9 g
Sodium sulphite, anhydrous	38.0 g
Sodium thiosulphate, anhydrous	0.9 g
Potassium carbonate, anhydrous	19.5 g
Potassium bicarbonate	13.3 g
Benzotriazole	1.0 g
1-Phenyl-4-methylpyrazolidone	0.5 g
Hydroquinone	8.0 g
Ethylcellosolve	57.4 g
Water to make up to	1,000.0 ml

TABLE 1

ADDITIONS TO KG A	EXPERIMENT NO
	1	2	3	4	5	6
AgNO₃ 1% [ml]	1.93	1.93	1.93	1.10	1.10	1.10
pAg	6.00	6.00	6.00	7.50	7.50	7.50
KBr 1% [ml]	8.50	8.50	8.50	7.10	7.10	7.10
Sensitiser^x [ml]	7.30	0.70	0.70	7.30	0.70	0.70
Sensitis time [min]	60	30	15	60	30	15
AgNO₃ 1% [ml]		11.3	11.3		10.0	10.0
pAg		6.0	6.0		7.5	7.5
KBr 1% [ml]		8.8	8.8		7.4	7.4
Sensitiser^x [ml]		6.6	0.7		6.6	0.7
Sensitis time [min]		30	15		30	15
AgNO₃ 1% [ml]			12.4			10.7
pAg			6.0			7.5
KBr 1% [ml]			10.0			8.3
Sensitiser^x [ml]			0.7			0.7
Sensitis time [min]			15			15
AgNO₃ 1% [ml]			14.1			11.8
pAg			6.0			7.5
KBr 1% [ml]			11.2			9.3
Sensitiser^x [ml]			5.2			5.2
Sensitis time [min]			15			15

^x 495mg of sensitiser of the formula (22) dissolved in 1,000ml of methanol.

TABLE 2

EXPERIMENT NO	NUMBER OF REPEATS	D-min	LOG E^x	LIRF^xx
Comparison 1	0	0.05	0.00	- 0.51
1	1	0.05	- 0.13	- 0.51
2	2	0.06	- 0.21	- 0.23
3	4	0.05	- 0.07	- 0.22
Comparison 2	4	0.05	- 0.10	- 0.50
4	1	0.05	- 0.17	- 0.51
5	2	0.05	- 0.10	- 0.13
6	4	0.05	0.00	- 0.08

^x measured at 50% of maximum density for 1 sec exposure
^xx LIRF = logE (1 second exposure) - logE (256 seconds exposure)

The results shows that an improvement in LIRF arises only if a part quantity of the sensitiser is added after each repeat, that at least two repeats are necessary, and that it is not necessary to generate an excess of silver ions in the emulsion.
Similar results are obtained if, in place of the sensitiser of the formula (22), the sensitisers of the formulae 18 to 21 or 23 to 29 are used. Similar results are also obtained if, in place of the silver bromoiodide emulsion, a pure silver bromide emulsion (polydisperse, 0.72nm mean crystal size), a silver chlorobromide emulsion (cubic monodisperse, 0.4µm mean crystal size, 70 mol% of chloride) or an emulsion with tabular silver halide crystals (0.70nm mean crystal size, 2.6 mol% of iodide) is used.

Example 2

Silver nitrate solution, potassium bromide solution and sensitiser of the formula (22) are added alternatingly at 40°C to 123g of the emulsion mixture A from Example 1, the number of cycles being varied from 4 to 7.
The experimental conditions are indicated in Table 3. 100ml of a 1.1% aqueous solution of the azo dye of the formula:

are then added. This mixture is, together with a gelatin supercoat which contains 1.5g of gelatin and 48mg of the gelatin hardener 2,4-dichloro-6-hydroxy-triazine (potassium salt) per m², coated onto a transparent polyester base (100ml per m²). The materials are exposed in the usual manner and processed at 30°C as follows:

1. Developing 3 minutes bath 1

2. Washing 1 minute

3. Bleaching 3 minutes bath 2

4. Washing 1 minute

5. Fixing 3 minutes bath 3

6. Washing 4 minutes
Bath 1 is the same as Developer A from Example 1, and baths 2 and 3 are of the following composition:

Bath 2 - Bleaching Bath

m-Nitrobenzenesulphonic acid	7.5 g
Sulphuric acid (100%)	41.8 g
Ethylcellosolve	57.4 g
2,3,6-Trimethylquinoxaline	1.1 g
Potassium iodide	9.0 g
bis-(βCyanoethyl)-sulphoethylphosphine	2.9 g
Water to make up to	1,000 ml

Bath 3 - Fixing Bath

Ammonium thiosulphate	200 g
Ammonium sulphite	17.9 g
Ammonium hydrogen sulphite	17.9 g
Water to make up to	1,000 ml

TABLE 3

ADDITIONS TO EMULSION MIXTURE A	EXPERIMENT NO
	1	2	3	4
AgNO₃ 5% [ml]	0.22	0.22	0.22	0.22
KBr 5% [ml]	1.42	1.42	1.42	1.42
Sensitiser (22)^x [ml]	0.70	0.70	0.70	0.70
Sensitis time [min]	15	12	10	9
AgNO₃ 5% [ml]	2.55	2.55	2.55	2.55
KBr 5% [ml]	1.84	1.84	1.84	1.84
Sensitiser (22)^x [ml]	0.70	0.70	0.70	0.70
Sensitis time [min]	15	12	10	9
AgNO₃ 5% [ml]	2.55	2.55	2.55	2.55
KBr 5% [ml]	1.84	1.84	1.84	1.84
Sensitiser (22)^x [ml]	0.70	0.70	0.70	0.70
Sensitis time [min]	15	12	10	9
AgNO₃ 5% [ml]	2.55	2.55	2.55	2.55
KBr 5% [ml]	1.84	1.84	1.84	1.84
Sensitiser (22)^x [ml]	0.70	0.70	0.70	0.70
Sensitis time [min]	15	12	10	9
AgNO₃ 5% [ml]		2.55	2.55	2.55
KBr 5% [ml]		1.84	1.84	1.84
Sensitiser (22)^x [ml]		4.50	0.70	0.70
Sensitis time [min]		12	10	9
AgNO₃ 5% [ml]			2.55	2.55
KBr 5% [ml]			1.84	1.84
Sensitiser (22)^x [ml]			3.80	0.70
Sensitis time [min]			10	9
AgNO₃ 5% [ml]				2.55
KBr 5% [ml]				1.84
Sensitiser (22)^x [ml]				3.70
Sensitis time [min]				9

^x 495mg of sensitiser of the formula (22), dissolved in 1,000ml of methanol.

TABLE 4

EXPERIMENT NO	NUMBER OF CYCLES	D-min	LOG E^x	LIRF^xx
1	4	0.04	0.78	- 0.37
2	5	0.04	0.78	- 0.37
3	6	0.04	0.83	- 0.36
4	7	0.04	0.75	- 0.40
Comparison^xxx	none	0.04	0.71	- 0.62

^x measured at 50% of maximum density for 1 sec exposure
^xx LIRF = logE (1 second exposure) - logE (256 seconds exposure)
^xxx in the comparison the same amount of sensitising dye was added as in Experiments 1 to 4 but the whole amount was added in one addition.

The sensitometric results given in Table 4 show that no differences arise between 4 and 7 repeats.

Example 3

123g of the emulsion mixture from Example 1 in each case are treated according to Table 5 with silver nitrate solution and potassium bromide, potassium iodide, potassium chloride or potassium thiocyanate and then with the sensitising dye of the formula:

4 cycles are carried out in each case, magenta dye is then added to the mixtures as indicted in Example 2 and coating on a polyester base is carried out.

The sensitometric results, after processing as indicated in Example 2, are summarised in Tables 5 and 6. It will be seen that all halides and thiocyanate lead to a substantial improvement in LIRF and, additionally, chloride treatment increases the sensitivity.

TABLE 5

ADDITIONS TO EMULSION MIXTURE A	EXPERIMENT NO
	1	2	3	4
AgNO₃ 1% [ml]	0.4	0.4	0.8	1.29
KBr 1% [ml]	6.0	-	-	-
KI 1% [ml]	-	8.37	-	-
KCl 1% [ml]	-	-	3.76	-
KSCN 5% [ml]	-	-	-	0.89
Sensitiser (31)^x [ml]	0.66	0.66	0.66	0.66
Sensitis time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.56	1.56	1.56	1.56
KBr 1% [ml]	1.19	-	-	-
KI 1% [ml]	-	1.66	-	-
KCl 1% [ml]	-	-	0.75	-
KSCN 5% [ml]	-	-	-	0.98
Sensitiser (31)^x [ml]	0.66	0.66	0.66	0.66
Sensitis time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.72	1.72	1.72	1.72
KBr 1% [ml]	1.20	-	-	-
KI 1% [ml]	-	1.69	-	-
KCl 1% [ml]	-	-	0.76	-
KSCN 5% [ml]	-	-	-	0.98
Sensitiser (31)^x [ml]	0.66	0.66	0.66	0.66
Sensitis time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.78	1.78	1.78	1.78
KBr 1% [ml]	1.27	-	-	-
KI 1% [ml]	-	1.77	-	-
KCl 1% [ml]	-	-	0.80	-
KSCN 5% [ml]	-	-	-	1.04
Sensitiser (31)^x [ml]	4.70	4.70	4.70	4.70
Sensitis time [min]	15	30	30	30

^x 673mg of the sensitiser of the formula (31), dissolved in 1,000ml of an n-propanol/water mixture (1+1).

TABLE 6

EXPERIMENT NO	ANION	LOG E^x	LIRF^xx
1	Bromide	0.70	- 0.31
2	Iodide	1.07	- 0.35
3	Chloride	0.39	- 0.44
4	Thiocyanate	0.74	- 0.26
Comparison	No cycles	0.70	- 0.62

Example 4

Four cycles with silver nitrate solution and potassium bromide solution are carried out on emulsion mixture A, as indicated in Example 3. However, the sensitiser dye is added once immediately after the silver nitrate solution (Experiment A) and the other time only at the end of the 4th cycle (Table 7, Experiment B). The emulsions are, as indicated in Example 1, coated onto a polyester base and processed.

Table 8 shows the sensitometric results. It will be clearly seen that, in both cases, there is no improvement in LIRF by reference to a comparison without cycles.

TABLE 7

ADDITIONS TO EMULSION MIXTURE A	EXPERIMENT
	A	B
AgNO₃ 5% [ml]	2.01	2.01
Sensitiser^x [ml]	0.70	-
Sensitising time [min]	15	15
KBr 5% [ml]	1.40	1.40
AgNO₃ 5% [ml]	2.03	2.03
Sensitiser^x [ml]	0.70	-
Sensitising time [min]	15	15
KBr 5% [ml]	1.44	1.44
AgNO₃ 5% [ml]	2.05	2.05
Sensitiser^x [ml]	0.70	-
Sensitising time [min]	15	15
KBr 5% [ml]	1.42	1.42
AgNO₃ 5% [ml]	2.02	2.02
Sensitiser^x [ml]	5.20	7.30
Sensitising time [min]	15	15
KBr 5% [ml]	1.41	1.41

^x measured at 50% of maximum density for 1 sec exposure

TABLE 8

EXPERIMENT NO	D-min	log E^x	LIRF^xx
A	0.66	- 0.18	- 0.48
B	0.06	- 0.19	- 0.48
Comparison	0.06	- 0.20	- 0.51

^x measured at 50% of maximum density for 1 sec exposure
^xx LIRF = log E (1 second exposure) - log E (256 seconds exposure)

Example 5

77g of a 2.5% aqueous gelatin solution and 32g of a polydisperse silver bromoiodide emulsion are mixed together at 40°C (emulsion mixture B). The silver halide emulsion contains 71g gelatin/kg and 57g of Ag/kg as silver bromoiodide with 5.2 mol% of iodide, and the mean particle size is 0.55µm. The silver halide emulsion has been chemically sensitised with thiosulphate and gold thiocyanate to optimum speed.
Silver nitrate solution, potassium bromide solution and a solution of the sensitiser of the formula (27) as indicated in Table 9 are added to this mixture alternatingly at 40°C, the number of cycles being 4. (Trial No 1 and 2). 100g of a colloidal dispersion of the barium salt of the cyan dye of the formula (III) are then added. The dispersion of the cyan dye contain 7g gelatin and 1.7g dye.

The mixture is, together with a gelatin supercoat containing 1.5g gelatin and 48mg of the gelatin hardener 2,4-dichloro-6-hydroxy-triazine (potassium salt) per m², coated onto a transparent polyester base (100ml per m²).
Both materials are exposed and processed as described in Example 2. The sensitometric results given in Table 10 shows that speed (Log S) is higher and LIRF is less than a comparison without cycles.

Example 6

The procedure is the same as described in Example 5, however instead of a solution of potassium bromide a solution of potassium thiocyanate is used. The experimental conditions are indicated in Table 9, the sensitometric results in Table 10 (trials 3 and 4).

TABLE 9

ADDITIONS TO EMULSION MIXTURE B	TRIAL NO
	1	2	3	4
AgNO₃ 1% [ml]	0.8	1.0	0.4	0.8
KBr 1% [ml]	6.0	6.0	-	-
KSCN 1% [ml]	-	-	4.9	4.9
Sensitiser (27)^x [ml]	1.19	1.19	1.19	1.19
Sensitising Time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.56	1.56	1.56	1.56
KBr 1% [ml]	1.19	1.19	-	-
KSCN 1% [ml]	-	-	0.97	0.97
Sensitiser (27)^x [ml]	1.19	1.19	1.19	1.19
Sensitising Time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.72	1.72	1.72	1.72
KBr 1% [ml]	1.21	1.21	-	-
KSCN 1% [ml]	-	-	0.99	0.99
Sensitiser (27)^x [ml]	1.19	1.19	1.19	1.19
Sensitising Time [min]	15	15	15	15
AgNO₃ 1% [ml]	1.78	1.78	1.78	1.78
KBr 1% [ml]	1.27	1.27	-	-
KSCN 1% [ml]	-	-	1.04	1.04
Sensitiser (27)^x [ml]	8.35	8.35	8.35	8.35
Sensitising Time [min]	30	30	30	30

^x 50mg of sensitiser of the formula (27), dissolved in 100g of an n-propanol/water mixture (1+1)

TABLE 10

TRIAL NO	D-min	LOG S^x	LIRF^xx
1	0.04	- 0.12	- 0.25
2	0.04	- 0.17	- 0.23
3	0.04	- 0.20	- 0.25
4	0.04	- 0.14	- 0.19
Comparison without cycles	0.04	+ 0.45	- 0.55

^x LOG S = log sensitivity at 0.5 maximum density for 1 sec exposure
^xx LIRF = LOG S (1 sec exposure) - LOG S (256 sec exposure)

This result shows that for the emulsions sensitised according to the present invention the speed (Log S) is higher and LIRF is less than the comparison.

Example 7

102g of a polydisperse silver bromide emulsion are heated to 40°C. The silver halide emulsion contains tabular crystals with an aspect ratio of 5 and a mean grain size of 0.9um. The emulsion has been chemically sensitised with thiosulphate and gold thiocyanate to optimum speed and contains 71g of gelatin/kg and 57g of silver/kg.
Silver nitrate solution, potassium bromide solution and sensitiser of the formula (23) are added alternatingly at 40°C to the silver halide emulsion, the number of cycles being 4. The experimental conditions are indicted in Table 11.

97g of a 5.5% aqueous gelatin solution and 203g of a 0.9% solution of the yellow dye of the formula (IV) are then added.

This mixture is, together with a gelatin supercoat which contain 1.5g of gelatin and 48mg of the gelatin hardener 2,4-dichloro-6-hydroxy-triazine (potassium salt) per m², coated onto a transparent polyester base (100ml per m²).

TABLE 11

ADDITIONS TO EMULSION	TRIAL NO
	1	2
AgNO₃ 1% [ml]	1.20	2.0
KBr 1% [ml]	18.0	30.0
Sensitiser (23)^x [ml]	5.6	5.6
Sensitising Time [min]	15	15
AgNO₃ 5% [ml]	3.10	3.10
KBr 5% [ml]	1.5	1.5
Sensitiser (23)^x [ml]	5.6	5.6
Sensitising Time [min]	15	15
AgNO₃ 5% [ml]	3.50	3.50
KBr 5% [ml]	1.5	1.5
Sensitiser (23)^x [ml]	5.6	5.6
Sensitising Time [min]	15	15
AgNO₃ 5% [ml]	3.60	3.60
KBr 5% [ml]	1.6	1.6
Sensitiser (23)^x [ml]	39.2	39.2
Sensitising Time [min]	30	30

^x 100mg of sensitiser of formula (23), dissolved in 100g of an n-propanol/water mixture (1+1).

TABLE 12

TRIAL NO	D-min	LOG S^x	LIRF^xx
1	0.04	0.52	- 0.28
2	0.04	0.67	- 0.20
Comparison without cycles	0.04	0.60	- 0.50

^x measured at 50% of maximum density for 1 sec exposure
^xx LIRF = log E (1 sec exposure) - log E (512 sec exposure)

The sensitometric results show that LIRF is considerably reduced.

Claims

A process for the spectral sensitisation of a silver halide emulsion characterised in that:
A) (1) A silver salt solution is added to a preformed silver halide emulsion until the pAg value reaches a point close to equivalence, (2) the pAg value of the emulsion is then adjusted to a value between 7.5 and 10, by addition of an ammonium or alkali metal halide or pseudo-halide solution to achieve fractional coverage of newly deposited silver halide, (3) a part quantity of a predetermined quantity of a solution of a sensitising dye capable of forming a J band is adsorbed on the newly deposited silver halide and:

B) stages (1), (2) and (3) are repeated at least once more but up to seven times, the residual quantity of sensitising dye being adsorbed with the last repeat of stage (3).
A process according to Claim 1 characterised in that in stage (2) the pAg value of the emulsion is adjusted between 7.5 and 8.5.
A process according to Claim 1 characterised in that stages (1) to (3) are repeated from 3 to 5 times.
A process according to Claim 1 characterised in that the preformed silver halide emulsion has been chemically sensitised.
A process according to Claim 1 characterised in that the pAg value in stage (1) is not more than one pAg unit above the equivalence point of the emulsion.
A process according to Claim 1 characterised in that an alkali metal thiocyanate solution is added in stage (2).
A process according to Claim 1 characterised in that an alkali metal chloride solution is added in stage (2).
A process according to Claim 1 characterised in that an alkali metal bromide solution is added in stage (2).
A photographic material which contains, on a base, at least one silver halide emulsion spectrally sensitised according to Claim 1.